Evidence of the defect-induced ferromagnetism in Na and Co codoped ZnO

The effect of Na concentration on the room-temperature ferromagnetism in Na and Co codoped ZnO diluted magnetic semiconductor (DMSs) was investigated. The ferromagnetic state was found to be stable below 5% doping of Na due to the exchange interaction via electron trapped oxygen vacancies (F-center) coupled with the magnetic Co ions. With large Na doping of up to 10%, a sharp reduction in the magnetization was observed, showing that the oxygen vacancy mediated antiferromagnetic state becomes predominant. The observed correlation between the Na concentration, the carrier concentration, and the magnetization demonstrated the effect of the defect in controlling the ferromagnetism in the ZnO-based DMS system.